The art of lithographic printing is based on the immiscibility of ink, generally an oily formulation, and water, wherein in the traditional method the ink is preferentially retained by the image or pattern area and the water or fountain solution is preferentially retained by the non-image or non-pattern area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water whilst the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
New types of "waterless" lithographic printing employ only an oily ink material and preferentially ink-accepting image areas and ink-repelling non-image areas on the printing form.
A generally used type of lithographic printing form precursor (by which we mean a coated printing form prior to exposure and development) has a light sensitive coating applied to an aluminium base support. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to radiation of a suitable wavelength hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. On the other hand positive working lithographic printing form precursors have a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. In both cases only the image area on the printing form itself is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing form precursor with a vacuum to ensure good contact. The printing form precursor is then exposed to a light source, comprising UV radiation. In the case where a positive form precursor is used, the area of the film that corresponds to the image in the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed.
The photoresists used in pattern forming methods for electronic parts such as printed circuits are also classified into two types: negative working and positive working. After exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements--for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive working systems are widely used. In particular, in the main there have been used alkali developable positive working photoresists mainly composed of alkali-soluble novolac resins as disclosed in J. C. Streiter, Kodak Microelectronics Seminar Proceedings, 1979, p. 116. The primary active component of such positive working compositions, both in the context of lithographic printing forms and electronic parts, is a naphthoquinonediazide (NQD) derivative.
The types of electronic parts whose manufacture may use a photoresist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
Imageable compositions may also be applied to masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing or electronic part substrate.
Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and NQD derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulphonyl chloride. The latter compounds were developed primarily as a solution to crystallisation and/or migration of simple NQD compounds within coated films, see, e.g., Reiser, Photoreactive Polymers, Wiley-Interscience, 1988. These NQD resin esters have also been used in the electronics industry to restrict migration of NQD compounds in multi-layer resist systems.
As demands on the performances of radiation sensitive coatings have increased UV-based NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on coatings in both lithography and electronic parts imaging.
One of the reasons for using NQD derivatives in UV systems is their sensitivity to UV radiation. The use of NQD derivatives in non-UV systems is not a natural step to take in response to the growing importance of laser imaging technology. However, there have been proposals to use NQD derivatives in laser systems which require an initial overall exposure to UV radiation.
PCT/GB95/02774 describes a method of forming a lithographic plate by a heat-mode imaging method which comprises coating on a substrate a positive working photosensitive composition which comprises a naphthoquinone diazide ester of a phenolic resin, or a naphthoquinone diazide ester and a phenolic resin, and at least one substance which absorbs infra-red radiation, overall exposing the assembly to UV radiation to render the photosensitive composition developable, imaging the plate by means of a laser which emits in the infra-red region of the spectrum and then developing the plate to remove those areas of the photosensitive composition not exposed to the laser.
U.S. Pat. No. 4,544,627 describes a composition containing an o-quinonediazide compound, and a method in which there is firstly an overall exposure to "actinic radiation," followed by an imagewise exposure to a laser beam. "Actinic radiation" is defined as "radiation to which the o-quinone diazide compound is sensitive to be converted to the corresponding indene carboxylic acid." It is said to be "generally, light having the wavelength from about 290 nm to about 500 nm although an electronic beam can also be used." On development only the regions of the coating unexposed to the laser beam are dissolved.
U.S. Pat. No. 5,631,119 discloses a process in which a composition containing an o-quinonediazide compound is exposed on its entire surface to light rays of wavelength about 290-500 nm to render the o-quinonediazide compound soluble in an alkaline developer, imagewise heating the composition, and then developing, to remove only those areas which have not been heated. Imagewise heating may employ infrared rays and an infrared-absorbing dye within the composition.
It will be noted that in each of these procedures the first step is an overall exposure of the coating to UV or near-UV radiation. Thus, although these are laser imaging systems the sensitivity of the NQD moieties to UV radiation is still utilized, by means of the preliminary overall UV exposure step required, and is an essential requirement of the procedures; save for the electron beam possibility additionally proposed (but not exemplified) in U.S. Pat. No. 4,544,627.
There have also been disclosures of positive working compositions containing quinonediazide moieties.
U.S. Pat. No. 5,200,292 discloses positive working compositions containing a resin and a) an aromatic diazo compound, for example a naphthoquinonediazide, and b) a cationic dye/borate anion complex. The anionic dyes exemplified have their peak absorbency from 428 nm to 740 nm.
U.S. Pat. No. 5,227,473 discloses positive working compositions containing novel compounds having a) a decomposable quinonediazide residue Q and b) a light absorbing portion S. The compounds have spectral sensitization with respect to visible light, which is said to be able to decompose the compounds efficiently. It is said to be important that the light absorbing portion S and the quinonediazide residue Q reside at very close separation. The examples employ light whose components of wavelength below 460 nm are filtered out.
We have now discovered to our surprise that diazide-containing compositions (which may be tried and tested compositions for UV systems) may be straightforwardly imaged in positive working methods by means of radiation, which is not the conventional UV radiation.